The present invention relates to a load modulation device in a remotely powered integrated circuit. Such a device provides data transmission between the remotely powered integrated circuit and a reader, and provides a source of an electromagnetic field by causing an equivalent load of the integrated circuit to vary, as seen from the reader. This invention is notably applied to contactless chip cards and to electronic labels or tags.
In such applications, an oscillating circuit of the LC type, for example, is used for regenerating the power supply and transmitting data between the card and the reader. The oscillating circuit may be partly or totally integrated into the integrated circuit or may be externally offset.
The oscillating circuit placed in an electromagnetic field delivers at its terminals an alternating signal with the same frequency as the signal emitted by the reader. The amplitude of this voltage signal is maximum when the resonant frequency of the oscillating circuit is equal to the emission frequency of the reader.
An integrated circuit for such applications usually comprises a circuit for rectifying the alternating signal provided by the oscillating circuit. The function of the rectifier circuit is to connect this alternating voltage to a continuous load for matching the load of the logic circuits of the integrated circuit. In other words, this rectifier circuit converts the power supply alternating voltage into a DC voltage for powering the logic circuitry of the integrated circuit.
The load modulation causing the impedance of the tuning circuit, as seen from the reader, to vary according to the data to be transmitted. The integrated circuit comprises for this purpose a load modulation circuit, controlled by a modulation logic signal delivered by a data transmission stage of the integrated circuit. The load modulation circuit generally includes one or more transistors connected between the output pads of the oscillating circuit, and is controlled by the modulation logic signal.
FIG. 1 illustrates a first exemplary embodiment of a load modulation circuit in a remotely power integrated circuit. This integrated circuit comprises conventionally, an oscillating circuit 1 which delivers an alternating voltage signal between its terminals A and B, and a rectifier circuit 2 for, rectifying this alternating voltage signal to provide DC power supply voltages Vdd and Gnd to the logic circuitry 3 of the integrated circuit. In the example, the rectifier circuit 2 is provided with a diode bridge D0, D1, D2, D3 and the logic circuitry 3 is illustrated by its equivalent load, with a resistor Re and a capacitor Ce parallel-connected between supply voltages Vdd and Gnd.
The load modulation circuit 4 of the oscillating circuit 1 is controlled by a modulation binary signal, marked as mod, and is delivered by a data transmission stage ED in the logic circuitry 3, not shown.
The load modulation circuit 4 comprises a switching transistor Tm1 controlled by the modulation signal mod on its gate. In the example, this is an N-type MOS transistor connected between a modulation node Nm and the power supply voltage Gnd. The load modulation circuit 4 further comprises two insulation transistors, one per terminal of the oscillating circuit, which protect the switching transistor Tm1 against voltages that are too high. There is an insulation transistor Ti1 connected between the terminal A and the node Nm, and an insulation transistor Ti2 connected between terminal B and node Nm. In the example, both of these are N-type MOS transistors. Each one is mounted as a diode with its gate and drain connected together.
When the switching transistor Tm1 is driven by the modulation binary signal mod into the off or blocked state, it is equivalent to a high value resistor which is marked as rdsoff. When the switching transistor is driven into the on or conducting state, it is equivalent to a low value resistor which is marked as rdson. The difference between resistors rdsoff and rdson generates the load variation. The operating angular pulsation of the oscillating circuit 1 remains unchanged.
FIG. 2 illustrates another exemplary embodiment of the load modulation circuit 4. In this example, the load modulation circuit comprises a capacitor Cm and a switching transistor Tm2 connected in series between terminals A and B. In the example, the switching transistor is an N-type MOS transistor, and receives the modulation binary signal mod on its gate. According to the binary level of the signal mod, the switching transistor Tm2 connects capacitor Cm in parallel to the oscillating circuit 1. Depending on whether the capacitor Cm is actually connected in parallel, the capacitor load and the operating pulsation of the oscillating circuit 1 are changed.
These load modulation circuit examples are state of the art, and in common they require connection of extra components to the oscillating circuit 1, which interferes with the quality factor of this circuit. The quality factor is understood as the overvoltage at its terminals at the circuit""s oscillation frequency. The current load due to extra components reduces the overvoltage, and therefore, the efficiency of the oscillating circuit 1.
In addition, extra components bring about a cost overrun in terms of implantation area on the integrated circuit. They should actually be able to withstand large voltage changes on the terminals of the oscillating circuit, which may attain 10 to 100 volts.
In view of the foregoing background, an object of the invention is to provide a load modulation device in a remotely powered circuit which does not have the above described drawbacks.
This and other objects, advantages and features are provided by using the parasitic drain/substrate diode or source/substrate diode of MOS transistors that are implemented in a well. According to the invention, by applying the modulation to the well of a MOS transistor that is connected through its drain or its source to a terminal of the oscillating circuit, the parasitic drain/well diode or source/well diode may become conductive. This has the effect of pulling the considered terminal up to a given voltage level, which amounts to modifying the load of the oscillating circuit, as seen from the reader.
As characterized, the invention therefore relates to a load modulation device in a remotely powered integrated circuit, and the device regenerates a first and a second power supply voltage for the circuit. The device comprises an oscillating circuit and at least one MOS transistor produced in a well, on at least one terminal of the oscillating circuit. The drain or the source of the at least one transistor is connected to the at least one terminal.
The invention is further characterized in that the modulation device comprises means or a circuit for biasing the well to the first or the second power supply voltage according to the level of a modulation binary signal.